1. Field of the Invention
This invention relates to heat transfer apparatus and, more specifically, to counterflow and parallel heat exchangers for heavy duty service, i.e., high pressure, or high pressure and high temperature applications with high fluid flow rates, especially in large utility power conversion systems operating on closed thermodynamic cycles and in some petrochemical installations and refineries, where the highest degree of heat recovery efficiently attainable is a prime consideration and the fluids are reasonably clean.
2. Description of the Prior Art
Heavy duty heat exchangers are almost always of shell-and-tube type because round tubes can accommodate very high fluid pressures and large thermal gradients. In flat plate and corrugated sheet heat exchanger matrices that capability is low and, therefore, they are not suitable for heavy duty application.
Shell-and-tube heat exchangers, however, commonly present many difficult problems in detail design; for example, provision for tube heaters. In service they frequently suffer significant performance degradation due to leakages at the tube to tubesheet joints and distortions of tube spacing resulting from thermal gradients and differential expansion between the tubes and the shell. They are frequently subject to severe vibration problems which may impose serious constraints upon their design. Their maintenance is costly due to the presence of many welds, which constitute the zones of stress concentration and are often susceptible to corrosion. These problems are particularly common and serious in high temperatures and high pressure applications in which differential thermal expansions are large, and tubes must be attached and sealed at the tubesheet by welding to achieve a high degree of leaktightness.
In applications with large fluid flow rates tubular heat exchangers are inherently bulky, complex and costly. Essentially, this is so because construction of counterflow tubular heat exchangers does not permit simultaneous employment of small diameter tubes and large fluid flow frontal areas. Tubes of small diameters are very difficult to install in thick tubesheets consistent with large fluid flow frontal areas. Furthermore, reductions in tube size must be accompanied by rapid increases in a number of tubes in order to maintain the required fluid flow rate, and by a decrease in tube spacing in order to maintain the proper balance between the thermal resistances on the tube side and the shell side of the heat exchanger. Thus, very soon, increases in the number of tubes becomes prohibitively costly both on account of manufacturing and material costs, since tube stock is relatively expensive. Furthermore, decrease in tube spacing soon becomes altogether impossible, since there is a limit upon the number of holes that can be drilled in a given tubesheet without the latter being critically weakened. For these reasons, even in the absence of corrosion and fouling, as in the case of clean fluids, relatively large tube sizes are necessary. As it is well known to those skilled in the art, increases in tube diameters require even greater proportional increases in tube length, if the required heat transfer effectiveness of the equipment is to be maintained. These, in turn, increase material costs, heat transfer losses, differential thermal expansion, and structural problems beyond tolerable limits.
Prior art includes many detail solutions aimed at alleviating the inherent disadvantages of shell-and-tube counterflow heat exchangers. For example, one ingenuous approach to the lack of space at both ends of shell-and-tube type heat exchanger is to group the tubes into bundles with large number of small headers. This arrangem:ent, originally worked out by Esher-Wyss, has been subsequently used quite commonly, although with some modifications, by others, e.g., Donald W. Culvert, U.S. Pat. No. 4,098,329. However, all such schemes do not solve the fundamental problem posed by the inability to provide small fluid passages in large, heavy duty heat exchangers; they merely alleviate it with some significant, yet limited degrees of success. It is largely for this reason that in many large installations, instead of one heat exchanger several smaller units working in parallel are frequently employed.
Another consideration that may limit the frontal area of a heavy duty shell-and-tube type heat exchanger is the wall thickness of the shell, particularly when the pressure of the shell side fluid is also high. The required wall thickness of cylindrical shell being proportional to its diameter, increasing shell diameter soon leads to excessive material costs, design difficulties, logistic problems and, possibly, potential safety hazards.
In some installations, such as shipboard power plants, restriction upon equipment length may dictate the use of tubes of the smallest size possible. In such cases, manufacturing costs grow rapidly, while excessive installation intricacies tend to degrade the system's reliability and performance. The ultimate effect of the four-way compromise which must be made between size, complexity, effectiveness and performance of the heavy duty heat exchange equipment is that the overall plant efficiency is significantly compromised, while its initial and operating costs attributable to the characteristics of shell-and-tube design still remain high.
Flat plate-fin and corrugated plate heat exchangers are very common in a great variety of forms. They can be both economical and highly satisfactory in some applications, albeit they also commonly experience difficult problems due to thermal gradients and susceptibility to vibrations. Quite often they are of annular cross-section as it is exemplified by the U.S. Pat. No. 3,228,464 to W. J. Stein et al, U.S. Pat. No. 3,741,293 to R. J. Haberski, U.S. Pat. No. 4,098,330 to R. J. Flower and others, this geometrical form being particularly well adaptable to aircraft gas turbines, which operate on open cycles, recover heat from exhaust gases and, therefore, require low pressure regenerators only. Torroidal heat exchanger disclosed in U.S. Pat. No. 3,255,818 to P. E. Beam, Jr., et al, may be of special interest in that it uses involute plate. U.S. Pat. Nos. 3,495,434 and 4,049,051 and 4,073,340 to K. O. Parker cover a gas turbine heat exchanger of the formed plate, counterflow type.
However, none of the prior art makes use of tapered plate as a structural or heat transfer element. Their assembly generally is an elaborate process. Furthermore, the common characteristic of all the aforementioned and other prior art plate heat exchangers is that their usefulness is limited to low pressure and low temperature systems or moderate pressure and temperature systems such that can tolerate certain amount of leakage between the hot and cold fluids. None of prior art plate heat exchangers qualifies nor is claimed to qualify for heavy duty, i.e., high temperature and/or pressure service such as is encountered, e.g., in some large utility power plants, gas cooled reactors, large petrochemical installations and some refineries.